In transporting and accurately metering long lengths of web material, a capstan roller with a pinch roller may be used as the metering device. When the web fed through the nip between the capstan roller and pinch roller with a web travel axis at 90.degree. to the capstan axis, and with upstream lateral guidance, the rolling action of the capstan will try to urge the web toward the 90.degree. approach angle. This action requires not only the lateral movement of the web on the roller, but also requires the rotation of the web in its own plane. That rotation causes a non-uniform velocity as measured across the width of the web as it passes over the capstan, which in turn, requires various degrees of slip and/or overdrive on the capstan. This action, which is restricted by the pinch roller, not only detracts from the accurate metering of the capstan, but requires the web to be highly tensioned and in danger of damage.
U.S. Pat. No. 4,395,152 which issued to Wolfgang Hendrisck on Jul. 26, 1983 discloses two friction rollers joined on a drive shaft with a differential being keyed to that shaft to cause one of the rollers that is not held to be driven. The rollers are ultimately held by, preferably, latching devices which are operated on limit positions of the print head. A differential is used to selectively drive or brake two coaxial rollers with one power source while transporting two webs. If the drive selector mechanism is removed, the action of the roller segments will allow web tracking corrections to be made. However, such a use is done in the presence of a strong steering torque which may damage the web. Accordingly, it will be appreciated that it would be highly desirable to have a web tracking correction mechanism that does not produce a dangerous amount of torque.
U.S. Pat. No. 4,335,971, which issued to Charles F. de Mey on Jun. 22, 1982, discloses a metering roller that is segmented to allow the web to rotate in its own plane at the roller for web alignment. The center segment of the roller is driven while the end segments are free to turn about the shaft at the same or different speeds. When a thin web is being transported and the center segment is driven, diagonal folds could develop from points in the web adjacent the driven segment to the edges of the web. It is also possible for nonuniform stretching to occur which would cause distortion in the web operation. Accordingly, it will be appreciated that it will be highly desirable to have a capstan mechanism that allows the web rotation in its plane for tracking alignment and also provide web drive in the entire roller length.